1. Field of the Invention
This invention relates to optical fibre arrangements in which one or more optical fibres or bundles of optical fibres provide illumination or collect radiation at the fibre ends.
2. Description of Related Art
Since an optical fibre has a circular cross-section, and assuming it terminates with a normal end-face, its angular response function (radiation characteristic) is circularly symmetric. The response falls off with increasing angle with respect to the fibre axis. FIG. 1 shows a typical response curve for one plane containing the fibre axis. It will be appreciated that by virtue of the symmetry essentially the same curve will be obtained for any plane containing the fibre axis. FIG. 2(a) shows a computer-generated 3D model of the characteristic and FIG. 2(b) a corresponding contour map. Similar characteristics are obtained for a bundle of fibres end, as used herein, any reference to an optical fibre is to be taken to include a bundle of optical fibres, unless the context requires otherwise. In FIG. 2(b), the contours represent steps of 10 per cent change from the peak response (at the fibre axis 15), the map axes indicating the angle a ray makes with the axis 15 in two orthogonal planes containing the axis 15. The apparent non-uniformities in the response for different planes containing the fibre axis are attributable to errors in the modelling process.
In many applications the circular symmetry in the radiation characteristic is undesirable. One such application uses an assembly of fibres to illuminate a region which is of greater extent than can be covered by a single fibre. However, the individual fibres must be arranged so that the outputs of neighbouring fibres overlap in order to ensure that there are no `holes`, i.e. areas of inadequate illumination, in the combined output. This problem arises whether the fibres are arranged parallel to one another, for example to illuminate a planar surface, or the fibres are angularly spaced so as to cover an angular field greater than that which can be covered by a single fibre. The situation is illustrated, by way of a simplified example, in FIG. 3, in which response contours 1,2,3 corresponding to 25% of the peak light output are shown for three equi-angularly spaced fibres. The shaded overlap areas 4 ensure that no significant part of the triangular region defined by the three fibre axes receives illumination at a level which is below 75% of the peak output of the individual fibres. Although the illumination may fall slightly below the 75 % level at the mid-points of the overlap areas 4, dependent on the individual fibre characteristics, it is otherwise substantially uniform at this level within the triangular region.